This invention relates to electronic reconnaissance devices and more particularly to an improved coherent electromagnetic energy emitter location device for detecting and locating radars and other electromagnetic emitters such as those in communication networks.
The problem of locating an object on the surface of the earth by the use of indirect measurements has a long history. Surveyors and navigators, for example, have always been concerned with location estimation.
To locate an object on the surface of the earth in a three dimensional coordinate system requires measurements on at least three independent quantities functionally related to the three coordinate values of the object. For locating electromagnetic emitters passively, the measurements generally taken are emitter altitude (from local topographic data) and either of the following: (1) direction of arrival of the electromagnetic waves at two or more locations of a single aircraft, or (2) time of arrival of emitter pulses at three aircraft locations. These measurements are then combined with the known (or estimated) locations from which they were taken, the functional relations among the various locations and measured quantities, and assumptions about the error distributions of the measurements, to arrive at an emitter location estimate.
Known systems include a single aircraft direction finding (DF) system which makes direction of arrival (directional bearing) measurements on emitter pulses from two or more locations along the aircrafts's flight path. If desired, the bearing measurement data may be combined with those of a second DF aircraft via a data link in order to obtain near instantaneous location estimates. The aircraft location and the bearing measurement base line are provided by a Loran-inertial navigation system or the like. In another system, time of arrival measurements are made on emitter pulses from three aircraft. The aircraft locations are determined from measurements of the ranges between aircraft and to two ground stations by distance measuring equipment systems, and aircraft altitudes from altimeters.
In the scenario in which there are exactly as many functionally independent quantities measured as there are coordinates to estimate, the location estimation problem is simply one of determining the solution of the functional equations. But, when there are more measurements than quantities to estimate, the question arises as to the proper method of combining the various measurements to obtain the most accurate location estimate as well as the probability distributions of the measurement errors and the relations among the measured quantities. Solutions for this problem depend upon the choice of criteria for the average closeness of an estimate as well as the probability distributions of the measurement errors and the relations among the measured quantities.
One standard measure of the closeness of a location estimation procedure is the location CEP (circular error probable), the circle around the true location within which 50% of such estimates would lie. When the emitter coordinate estimates have a multivariate normal distribution with mean values equal to the true coordinates, the location CEP can be expressed as a function of the variances and covariances of the location coordinate estimates. The lower bound is derived by use of the Cramer-Rao inequality under the assumption that the measurements have a multivariate normal distribution with mean values equal to the quantities measured. The covariance matrix for generalized least squares location estimates approximates that given by the lower bound. The emitter location CEPs are obtained for DF systems and time of arrival. A discussion of the above prior art is set forth in detail in DTIC Technical Report. ON THE ACCURACY ANALYSIS OF AIRBORNE TECHNIQUES FOR PASSIVELY LOCATING ELECTROMAGNETIC EMITTERS, by L. H. Wegner, R-722-PR, June 1971.
The disadvantage attending single-aircraft DF system, where direction of arrival information is obtained along the aircraft's flight path, is that the emitter to be located may cease transmitting. Thus, the solution to this problem has been to use two aircraft for the DF system and three aircraft for the time of arrival system.